Should I join a character and its accessories into one object for an animation?
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There's no real right or wrong answer here. If your character is really low poly, and all the accessories are permanently attached, then joining the objects is fine. However, keeping the separate also works well.

The advantage to keeping things separate is it allows you to edit any object at any point. It also makes assigning materials easier, and can even make rigging a bit easier. The reason for this is you can parent an object directly to a bone. This is really useful for objects like a scuba tank or mask that won't have to deform.

At the end of the day, I'd just pick one and give it a shot. You can always separate or combine objects if you think another way would work better. We're always here for any specific questions you may have along the way too.